Quantum cascade lasers (QCLs) differ from traditional semiconductor diode lasers in that they do not use a p-n junction for light emission. Multiple active regions are “cascaded” so that each injected electron can emit multiple photons and therefore enhance laser gain. Each active region includes a multi-layered semiconductor material structure. This multi-layered semiconductor material structure is designed to have an electronic band structure that gives the desired emission wavelength, and is manufactured with nanometer-level thickness control.
Most commercially available QCLs are of the “edge-emitting” variety. In these, linear ridges are created by etching vertically through the layered structure, which has been grown on a wafer substrate. These are cleaved so that the active region comprises a rectangular ridge several millimeters in length, which forms a waveguide. The laser radiation is amplified by passing back and forth along the axis of the ridge. Laser emission occurs on the end facets of these ridges.